1. Field of the Invention
The present invention relates to an optical fiber manufacturing apparatus and an optical fiber manufacturing method which can be suitably used in an optical fiber drawing process using a rod-in-tube method.
Priority is claimed on Japanese Patent Application No. 2008-311219 filed Dec. 5, 2008, the contents of which are incorporated herein by reference.
2. Description of the Related Art
A method of sintering and vitrifying a porous silica preform produced by a soot method such as a VAD method or an OVD method is generally used in manufacturing an optical fiber. However, with the recent increase in size of the optical fiber preform, there is a need for a method with higher productivity.
As such a method, a rod-in-tube method of manufacturing a cladding layer, which occupies most of an optical fiber, by the use of a jacket of an optical-fiber silica glass tube, is known. This method is roughly classified into two types: a method of obtaining an optical fiber preform by inserting an optical-fiber core rod into an optical-fiber silica glass tube and then collapsing the tube onto the core rod in a heating furnace; and a method of obtaining an optical fiber at a time by drawing an optical-fiber core rod and an optical-fiber silica glass tube while collapsing the tube onto the core rod. Here, the optical-fiber core rod can be produced using a known production method such as a VAD method, an MCVD method, and an OVD method and generally includes a core portion transmitting light and a part of a cladding layer.
When the collapsing is carried out in a heating furnace, a seal member is necessary which serves to prevent outer gas from flowing into a muffle tube through a gap between the muffle tube and an optical fiber perform. This is because a heater or a muffle tube in the heating furnace is generally made of carbon. When the sealing ability (air tightness) between the muffle tube and the optical fiber preform is poor, oxygen intrudes into the muffle tube from the outside, thereby causing the degradation or burnout of carbon components. The seal member is used to prevent oxygen from intruding into the muffle tube from the outside.
In the former method of obtaining the optical fiber preform by the collapsing in the heating furnace, a gas seal for preventing gas from intruding into the muffle tube by allowing purge gas of 30 to 100 SLM to flow is employed.
However, in the latter method of obtaining the optical fiber at the time of the drawing together with the collapsing, the gas flow in the muffle tube is important and a gas seal using a large amount of purge gas causes a variation in fiber diameter. Accordingly, it is necessary to seal a gap between the muffle tube and the optical fiber without using the gas seal.
However, in the rod-in-tube using a large-sized optical-fiber silica glass tube with a diameter of φ100 mm or more, a dummy silica tube is generally jointed to an end of the optical-fiber silica glass tube. By jointing the dummy silica tube, it is possible to draw an optical fiber up to the end of the optical-fiber silica tube. In addition, the dummy silica tube can be used as a grip for supporting the optical-fiber silica glass tube with a weight. In view of the cost, the dummy silica tube is smaller in outer diameter and thickness than the optical-fiber silica glass tube within a range where the strength can be maintained. The dummy silica tube may be reused and thus the outer diameter may decrease with an increase in the number of times used. That is, when the dummy silica tube is repeatedly reused, a difference in outer diameter is necessarily caused between the optical-fiber silica glass tube and the dummy silica tube. When an optical fiber is drawn up to the end of the optical-fiber silica glass tube, the jointed portion between the optical-fiber silica glass tube and the dummy silica tube is put into the muffle tube in the last stage of the drawing process.
However, as described above, since the difference in outer diameter exists between the optical-fiber silica glass tube and the dummy silica tube, the sealing ability at the muffle tube is varied during the drawing. Accordingly, the gas flow in the muffle tube varies, thereby causing a variation in fiber diameter. As a result, a problem of a decrease in the yield of the optical fiber or a degradation of carbon components due to the intrusion of oxygen occurs.
To solve the above-mentioned problem, various devices for improving the sealing ability between the muffle tube and the optical fiber preform have been proposed.
In an optical fiber drawing furnace disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-183045 (hereinafter, referred to as Patent Document 1), the inside of the muffle tube is maintained at a slight positive pressure using a carbon sheet having a hole corresponding to the diameter of the optical fiber preform and a gas seal together, thereby preventing ambient air from intruding and preventing dust from flying from the inside of the muffle tube.
In an optical fiber drawing apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2005-8475 (hereinafter, referred to as Patent Document 2) in which a carbon film is used as a gas seal member, even when the difference in outer diameter exists between a preform and a handle, gas is prevented from intruding into the muffle tube from the outside by using separate gas seal mechanisms for the perform portion forming an optical fiber preform and the handle portion jointed to the upper end of the perform portion.
In an optical fiber drawing apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-342030 (hereinafter, referred to as Patent Document 3), even when a difference in outer diameter exists in the optical fiber preform, the air tightness in the muffle tube can be maintained by using an extendable seal ring as a gas seal member for sealing an opening into which the optical fiber preform is inserted.
In an optical fiber drawing apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2007-70189 (hereinafter, referred to as Patent Document 4), gas is prevented from intruding into the muffle tube from the outside by using a carbon brush of a doughnut disk shape, in which heat-resistant hairs are densely implanted, as a gas seal member for sealing an opening into which the optical fiber preform is inserted.
However, the optical fiber drawing furnaces and apparatuses disclosed in Patent Documents 1 to 4 have the following problems.
Specifically, in the optical fiber drawing furnace disclosed in Patent Document 1, the carbon sheet having a hole corresponding to the diameter of the optical fiber preform is used together with the gas seal, in which the inner diameter of the hole formed in the carbon sheet is constant. Accordingly, with a variation in the diameter of the optical fiber preform, a gap is caused between the optical fiber preform and the hole of the carbon sheet. As a result, since the pressure in the muffle tube varies, it is necessary to adjust the gas flow rate so as to correct the variation in pressure. However, the variation of the gas flow rate during the drawing causes a variation in the fiber diameter, which is not desirable.
In the optical fiber drawing apparatus disclosed in Patent Document 2, the gas seal mechanisms are used in the preform and the handle, respectively, to cope with the difference in outer diameter and a flat disk or a cylinder for loading a carbon film, a seal ring, and a weighting ring serving as a weight of the carbon film are necessary for the gas seal of the handle. However, since the members made of metal or silica glass are heavy, the workability for adjusting the levelness of the carbon film is poor. When a material equal to or harder than glass is used, the optical fiber preform may be damaged by mistake.
In the optical fiber drawing apparatus disclosed in Patent Document 3, the extendable seal ring is used as the gas seal member. However, the seal ring includes plural seal ring pieces bonded to each other. Accordingly, since the extension and contraction of the seal ring pieces due to heat are inevitable, the seal ring pieces may not be accurately bonded to each other. Since the bonding accuracy between the seal ring pieces is poor, a gap is caused between the bonded seal ling pieces, thereby degrading the sealing performance. An extension spring for extending and contracting the seal ring pieces may fail to work because of degradation due to heat. In addition, dust generated by the oxidation due to heat may intrude into the extension spring, thereby causing the spring to not work smoothly.
In the optical fiber drawing apparatus disclosed in Patent Document 4, the carbon brush of a doughnut disk shape is used as the gas seal member and the heat-resistant hairs are densely implanted in the carbon brush. Accordingly, a large amount of powders generated in use may be adhered to the optical fiber preform, thereby causing fiber breaking.
As described in the patent documents, when the carbon sheet is used as the seal member, it is necessary to form an opening greater than the maximum diameter of the optical fiber preform so as not to bring the inner circumferential edge of the opening into direct contact with the optical fiber preform. However, in this case, when the outer diameter of the optical fiber preform decreases with the drawing of the fiber, there is a problem in that the gap between the opening of the carbon sheet and the optical fiber preform increases, thereby not maintaining the sealing ability. On the other hand, to secure the sealing ability, the use of a carbon sheet having an opening smaller than the minimum diameter of the optical fiber preform can also be considered. However, in this case, the outer circumferential surface of the optical fiber preform and the opening of the carbon sheet come in contact with each other and slide relative to each other, thereby damaging the carbon sheet.
In the rod-in-tube method using a large-sized optical-fiber silica glass tube with a diameter of φ100 mm or more, the dummy silica tube is generally jointed to the end of the optical-fiber silica glass tube. As described above, it is preferable in view of cost that the dummy silica tube be smaller in diameter than the optical-fiber silica glass tube. However, since the difference in outer diameter exists between the optical-fiber silica glass tube and the dummy silica tube, the sealing ability of the seal member may be degraded by the entrance of the dummy silica tube into the muffle tube.
As a result, ambient air intrudes into the furnace to cause the variation in fiber diameter at the time of the drawing and to degrade the carbon components (such as a heater).